The term "microwave assisted chemistry" refers to the use of electromagnetic radiation within the microwave frequencies to provide the energy required to initiate, drive, or accelerate certain chemical reactions. As chemists have long been aware, the application of heat energy is one of the most significant factors in increasing the rate of a wide variety of chemical reactions. Thus, generally familiar devices such as the Bunsen burner, other types of gas burners, hot plates, and other similar devices have historically been used to initiate or accelerate various chemical reactions.
As a relatively crude comparison, microwave assisted chemistry techniques are used to heat chemical reagents in the same way that a consumer microwave oven cooks food. There are significant differences, however, between the ordinary consumer use of microwave energy with food and its laboratory use with chemical reagents. Thus, the devices and techniques required for microwave assisted chemistry are generally much more sophisticated than are the consumer-oriented devices and techniques.
In one comparison, however, a laboratory microwave device and a consumer microwave offer the same advantage: in many circumstances they both greatly increase the rate at which materials can be heated as compared to the rates that they could be heated by ordinary conduction or convection heating. Thus, microwave assisted chemistry has been particularly valuable in driving or accelerating reactions that tend to be time-consuming under more conventional heating techniques. Particular examples include moisture analysis, in which samples must effectively be heated to dryness; digestion, a process in which a chemical composition is broken down into its elements for further analysis, with the breakdown generally being accomplished by heating the composition in one or more mineral acids; and the Kjeldahl techniques for nitrogen determination. Using conventional heating techniques, moisture analysis, Kjeldahl, or digestion reactions can be very lengthy, extending for hours in some cases. When the reactions are microwave assisted, however, they can be completed in a much shorter period of time. It will be understood that this time savings has a particularly significant advantage in any situation in which large number of samples must be tested on an almost continuous basis. Thus, although microwave assisted chemistry is relatively new compared to some other techniques, it has become well established and accepted in a number of analytical applications.
As well understood by those familiar with the electromagnetic spectrum, the term "microwave" is often used generically to refer to radiation with wavelengths of between about 1000 and 500,000 microns (.mu.), and corresponding frequencies of between about 1.times.10.sup.9 and 5.times.10.sup.11 Hertz (Hz). These are arbitrary boundaries, however, and other sources refer to microwaves as having frequencies of between about 10.sup.8 Hz and 10.sup.12 Hz and wavelengths of between about 300 centimeters (cm) and 0.3 millimeters (mm). For commercial and consumer purposes in the United States, the available microwave frequencies are regulated by the Federal Communications Commission and are generally limited to certain frequencies such as 2450 megahertz (MHz). Because of the relatively long wavelength of microwave radiation, microwave assisted chemistry techniques are often carried out in closed vessels which are in turn placed inside a device that bears a superficial relation to a consumer microwave oven, but that is much more sophisticated in its source, waveguide, cavity, and control elements.
In turn, because the reactions are often carried out inside closed vessels, and because the reactions often generate gas, the reactions tend to generate and build up significant pressure in the reaction vessels. Accordingly, vessels have been developed to withstand most expected pressures, and also to include various pressure relief devices to prevent the vessels from exploding under the significant pressures being generated. An exemplary vessel and pressure release system is set forth, for example in U.S. Pat. No. 5,369,034, which is assigned to CEM Corporation of Matthews, N.C.
Although the simple application of microwave energy to devices in sealed vessels has some advantages, the technique becomes particularly useful when the reactions inside the vessels can be monitored while microwaves are being applied. Thus, in a typical microwave assisted chemistry system, a plurality of similar reactions are carried out at the same time in separate closed vessels that are placed together in a single cavity and then concurrently exposed to microwaves from a single source. Typically, one of the vessels carries temperature and pressure measuring devices. This "sensor vessel" is monitored and the conditions therein are assumed to be representative of the conditions in the remainder of the vessels to which microwaves are being applied.
Stated differently, in certain microwave assisted systems, a group of reaction vessels (typically six or eight) is placed into the microwave device at the same time, and often on a turntable that rotates as the microwaves are being applied. As noted above the wavelength of microwaves is typically larger than the items being heated, so that stationary items are not always evenly exposed to the microwaves. Accordingly, smaller items such as reaction vessels and relatively small amounts of chemical reagents are best moved on a periodic basis while being exposed to the microwaves. For similar reasons, consumer kitchen microwave ovens typically include fan-like stirrers to more evenly reflect microwaves within a cavity, or turntables for rotating food as it cooks. Alternatively, microwave cooking instructions typically tell the consumer to turn, stir, or otherwise move the food during the cooking process.
During microwave assisted chemistry, pressure is generally monitored for safety purposes; i.e. to make sure that the pressure generated by the chemical reaction remains within the pressure-containment limits of the device. A typical technique incorporates flexible tubing that runs from inside the vessel to an external pressure measuring device. Such an arrangement has been generally satisfactory for earlier generations of microwave assisted devices and vessels that operated at relatively lower pressure; e.g., about 200 pounds per square inch (psi). Vessels are now available, however, that can operate at internal pressures of 600, 900, or even 1500 psi. The typically available tubing materials cannot withstand such higher pressures and thus previous pressure measurement techniques cannot match the improvements in the vessels.
Additionally, measuring the gas pressure inside the vessel fails to take into account other stresses that can affect the vessel, particularly the stresses resulting from thermal expansion.
Pressure can be measured, of course, by placing a pressure-measuring device inside the reaction vessels along with the chemical reagents and then monitoring the reactions as they proceed. Although conceptually attractive, internal measurement is limited by the frequent presence of concentrated mineral acids such as hydrochloric (HCl), sulfuric (H.sub.2 SO.sub.4) and phosphoric (H.sub.3 PO.sub.4) that are often used in microwave assisted chemistry. Because of their aggressive chemical natures, these acids tend to attack almost all other materials and very few types of measuring devices (whether for pressure, temperature, or other parameters) can withstand such attack on a repeated basis.